drm/i915: initialize the parity work only once

This fixes an (albeit really hard to hit) race resulting in an oops:
- The parity work get scheduled.
- We re-init the irq state and call INIT_WORK again.
- The workqueue code tries to run the work item and stumbles over a
  work item that should be on it's runlist.

Also initiliaze the work item unconditionally like all the others,
it's simpler.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>